Projection type image display apparatuses such as a liquid crystal projector use a light source unit which includes a high-intensity arc tube operable at a high temperature, such as a high-pressure mercury lamp or a metal halide lamp, enclosed by a reflector having a parabolic or ellipsoidal reflecting surface for reflecting the light from the arc tube in one direction through a front opening of the reflector. Under a normal operating condition, i.e. when the projector is installed on the level, there will be a temperature gradient inside the reflector such that the temperature is higher in the upper section of the reflector than in the lower section. Therefore, the arc tube is normally designed in such a way that its upper section is cooled harder than the lower section.
However, such projector type image display apparatus is often installed at different angular positions. For example, it may be installed upside down or upwardly (or downward) inclined to project images in the upward (downward) oblique direction. In that event, the “lower section” is not desirably cooled. Moreover, it is necessary to regulate the temperature of the arc tube within an appropriate temperature range, since the full performance of the arc tube cannot be elicited if its temperature is too high or too low.
Thus, conventional reflectors are mostly provided in one side thereof (with respect to the front opening of the arc tube arranged at a normal level position) with an air inlet port for taking in cooling air, and with an air outlet port in the opposite side facing the air inlet port, thereby ensuring uniform circulation of the cooling air in the upper section as well as in the lower section of the arc tube so as to minimize the temperature gradient even when the light source unit is installed upside down.
The Japanese Patent Application Laid Open No. H5-135746 discloses a technique for preventing temperature rise of the lowest temperature section (or the bulb-shaped light emitting section) of an arc tube caused by stagnation of the cooling air, in which the arc tube is provided with three air outlet ports one for each of the three sides other than the side having an air inlet port, and with an air blast nozzle at one end of the lowest temperature section.
However, in the conventional (i.e. commonly used) arrangements, cooling air fails to circulate round the entire arc tube, so that when the light source unit is installed at an arbitrary angular position, cooling air fails to adjust itself to cool the lowest temperature section that has shifted, that is, cooling air cannot fully regulate the temperature of such arc tube. Consequently, the arc tube is permitted to have only limited angular positions on account of the poor temperature controllability.
Moreover, the prior art light source unit as disclosed in the cited literature not only requires a blast nozzle, which in turn requires an extra cost therefor, but also complicates the configuration of the light source unit. In addition, since the blast nozzle is normally designed to blow air only to the upper section of the light source unit, it limits the range of the allowable angular positions of the arc tube